| Borehole apparatus and methods for simultaneous multimode excitation and reception to determine elastic wave velocities, elastic modulii, degree of anisotropy and elastic symmetry configurations -> Monitor Keywords |
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  Borehole apparatus and methods for simultaneous multimode excitation and reception to determine elastic wave velocities, elastic modulii, degree of anisotropy and elastic symmetry configurationsRelated Patent Categories: Communications, Electrical: Acoustic Wave Systems And Devices, Seismic Prospecting, Well LoggingThe Patent Description & Claims data below is from USPTO Patent Application 20070183259. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] This invention is related to measurements of properties of earth formation penetrated by a well borehole. More particularly, the invention is related to a well borehole system that generates and detects acoustic energy of multipole modes that is processed to determine a plurality of formation properties of interest including elastic properties of the formation characterized as an inhomogeneous anisotropic solid. BACKGROUND OF THE INVENTION [0002] Acoustic well logging comprises the measure of various acoustic properties of formation penetrated by a well borehole. These measured properties are subsequently used to determine formation and borehole properties of interest including, but not limited to, formation porosity, formation density, stress distribution, formation fracturing, and formation anisotropy. [0003] Elastic anisotropy manifests itself as the directional dependence of sound speed in earth formation. Anisotropy in earth formation may be due to intrinsic microstructure such as the case in shales, or may be due to mesostructure such as fractures, or may be due to macrostructure such as layering due to sedimentation. Whatever the cause for anisotropy may be, good estimates of elastic properties of anisotropic media are required in resolving seismic images accurately, in interpreting borehole logs and in estimating drilling mechanics parameters. Specifically, seismic lateral positioning, amplitude versus variation with offset (AVO) and vertical seismic profiling (VSP) interpretation, borehole stability and closure stress estimates are all dependent on an accurate and precise measure of the degree and configuration of anisotropy of subsurface formations. Even though prior art acoustic logging systems, such as the crossed dipole systems, have advanced the state of the art, the present state of borehole acoustic logging has not been able to measure the structure of elastic anisotropy in a consistent manner. [0004] Prior art multimode acoustic logging systems are typified by acoustic transmitter excitation with radiation patterns of 2n.sup.th order poles such as monopole (n=0), dipole (n=1) and quadrupole (n=2) as well as reception by a plurality of receivers with similar discrimination patterns. Examples of prior art systems are disclosed in Aron et al, "Real-Time Sonic Logging While Drilling in Hard and Soft Rocks", Paper HH, SPWLA 38.sup.th Annual Logging Symposium, 1997; Tang et al, Chapter 5, "Quantitative Borehole Acoustic Methods", Elsevier, 2004; Varsamis et al, "LWD Shear Velocity Logging in Slow Formations Design Considerations and Case Histories", SPWLA 41.sup.st Annual Logging Symposium, 2000; U.S. Pat. No. 5,753,812 "Transducer for Sonic Logging While Drilling", Aron, J., et al; and U.S. Pat. No. 6,213,250 "Transducer for Acoustic Logging, Wiesniewski, L. et al. Accordingly, a monopole measurement system will typically comprise one or more monopole transmitters and monopole receivers, and a dipole measurement system will typically comprise one or more dipole transmitters and dipole receivers. Prior art indicates, however, that when the axis of a borehole logging tool axis is eccentered or tilted with respect to the borehole axis, other undesired modes are also created in the borehole/formation system thereby contaminating the measurements of interest (see Leslie et al, "Eccentric Dipole Sources in Fluid-Filled boreholes: Numerical and Experimental Results", Journal of the Acoustical Society of America, Vol. 87, No., 6, pp. 2405-2421, 1990). The ability of the receivers to discriminate and filter out the unwanted modes is compromised by azimuthal aliasing as well as any phase and sensitivity mismatch of the plurality of receivers. [0005] Prior art multipole acoustic logging systems comprise separate transducer systems for excitation and detection of each order multipole. Such a system is disclosed by Pistre et al., "A Modular Wireline Sonic Tool for Measurements of 3D (Azimuthal, Radial, and Axial) Formation Acoustic Properties", SPWLA 46.sup.th Annual Logging Symposium, Jun. 26-29, 2005. [0006] There is no known acoustic logging system that can efficiently generate and sense multiple modes simultaneously without unacceptable degradation in performance due to eccentering, tilt or receiver phase mismatch. [0007] The prior art directed toward multipole mode acoustic logging systems is typically complex and is often impractical in a real-time borehole logging system. In addition, inaccuracies inherent in practical borehole measurements and the accuracy of required unknowns, such as formation elastic moduli or stress conditions, do not warrant such system complexity. As an example, a method commonly used in determining acoustic wave velocities (or slownesses), namely, the semblance technique, is more influenced by the group velocity rather than the phase velocity. In anisotropic media, the group and phase velocity vectors do not necessarily coincide thereby leading to measurement inaccuracies of indeterminate order. A prior art method for evaluating elastic wave velocities in anisotropic formations from borehole logging is disclosed in U.S. Pat. No. 6,772,067. This is a typical method employed in wireline acoustic crossed dipole logging. However, the performance of method is sensitive to the effects of eccentricity, tilt and receiver mismatch. BRIEF SUMMARY OF THE INVENTION [0008] The invention is an acoustic well borehole logging system directed toward the generation and detection of multipole modes for determination of elastic properties of earth formations characterized as inhomogeneous anisotropic solids. More specifically the system concurrently generates and senses monopole, dipole, quadrupole and any higher order pole in the borehole/formation system in order to characterize the elastic properties and stress state of formation penetrated by the borehole. Multipole modes of all orders, such as monopole, dipole, quadrupole, etc., are induced simultaneously without the need for separate transmitter and receiver systems. Furthermore, performance acceptability is not compromised due to eccentering of the axis of the tool in the borehole, tool tilt with respect to the axis of the borehole, or mismatch of response sensitivity of multiple receivers within the tool. The system comprises apparatus for generating and sensing acoustic signals in a borehole in an earth formation, and further comprises a processing method by which the sensor response signals are processed and analyzed to obtain desired formation parameters of interest. [0009] The system apparatus comprises a downhole instrument or borehole "tool", surface equipment, and conveyance system comprising a data conduit and conveyance means for conveying the tool along the borehole and operationally connecting the tool with the surface equipment. Tool response data are conditioned and processed in a tool conveyed processor or in a processor disposed within the surface equipment to obtain formation and borehole parameters of interest. The conveyance means can comprise a wireline, or a tubular such as coiled tubing, or a drill string. The tool can alternately be embodied as a "pump-down" system, which is conveyed along the borehole by drilling fluid. In the pump-down embodiment, acoustic receiver data responses are recorded as a function of pump-down tool position within the borehole. The pump-down tool is subsequently retrieved at the surface of the earth, and the receiver response data are extracted from the tool and transferred to the surface equipment via the data conduit operationally connecting the tool to the surface equipment. Parameters of interest are then obtained by processing the data in the surface equipment. [0010] The borehole tool comprises at least one acoustic impulse transmitter and a receiver array comprising at least two receiver stations spaced axially at predetermined distances from the transmitter. Each receiver station contains at least two azimuthally separated receiver elements at each station. The axial receiver station spacings and the azimuthal receiver element spacings around the axis of the tool need not be uniform. In the preferred embodiment the system is operated by firing the acoustic impulse transmitter with an amplitude weighting and phase weighting in the azimuthal direction to simultaneously generate all orders of multipoles that propagate along the borehole/formation system. Acoustic energy arriving at each element of the receiver array is finely sampled and digitized as a function of time. Each order pole at each receiver station is then extracted simultaneously by angular decomposition, and their velocities are determined across the same receiver array. [0011] For purposes of disclosure, it will be assumed that the system is embodied as a wireline system and that the tool comprises a mandrel in which at least one transmitter and a receiver array are disposed. The preferred embodiment of the system comprises a broadband acoustic impulse transmitter, disposed on the mandrel and operable at selected frequencies to generate at least one mode, such as a dipole, in the borehole-formation environs. The receiver array, axially spaced on the mandrel from the one or more transmitter, then senses the induced headwaves in the borehole associated with the waves in the formation, as well as the guided modes along the borehole. A processor cooperating with the receiver array finely samples and records the data for processing. [0012] The preferred method for conditioning and processing receiver array response is briefly summarized in the following steps: [0013] (1) The digitization of a finely sampled pressure field over the receiver array. [0014] (2) The identification of the propagating phases over the receiver array into each of the multipole modes such as monopole, dipole, quadrupole etc by an angular decomposition. [0015] (3) The calculation of the velocities (or slownesses) of each of the mode components including monopole velocities (or slownesses), dipole velocities (or slownesses), quadrupole velocities (or slownesses), and the like. [0016] (4) The identification of components of each multipole mode, such as monopole phase velocities that yield compressional and shear headwaves and Stoneley components if they exist. Dipole phase velocities will reveal whether or not there is birefringence due to anisotropy; and so on. [0017] (5) The use of the results of steps (3) and (4) to solve the inverse problem to obtain the in the elastic moduli and geophysical properties of the formation as a function of depth in a local coordinate system associated with the borehole. [0018] (6) The transformation of the results from step (5) into global coordinates of a global coordinate system associated with the location of the particular exploration or the development well being logged. [0019] (7) The use of the results of steps (4), (5) and (6) to obtain the elastic state of the formation as a function of position within the borehole, wherein the position is stated in the global coordinate system. BRIEF DESCRIPTION OF THE DRAWINGS [0020] So that the manner in which the above recited features, advantages and objects the present invention are obtained and can be understood in detail, more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. [0021] FIG. 1A conceptually illustrates the major elements of the acoustic logging system operating in a borehole environment; Continue reading... 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